Neovim, OCaml Interfaces, Tree-Sitter and LSP

Can we all agree that witnessing syntax highlighting being absolutely off is probably the most annoying thing that can happen to anybody?

I mean, just look at this horror.

What you are looking at is the result of trying to enable tree-sitter for OCaml hacking and calling it a day. In a nutshell, OCaml mli files are quickly turning into a random mess of nonsensical colors, and I didn’t know why. I tried to blame tree-sitter-ocaml , but, of course I was wrong.

The issue is subtle, and to be honest, I don’t know if I totally grasp it. But from my rough understanding, it breaks down as follows.

• tree-sitter-ocaml defines two grammars: ocaml for the ml files, and ocaml_interface (but ocamlinterface also works) for the mli files
• By default, neovim uses the filetype ocaml for mli files, so the incorrect parser is being used for syntax highlighting. This explains the root issue
• Bonus: ocamllsp does not recognize the ocamlinterface filetype by default (but somehow use the ocaml.interface id for mli files…There is probably something to be done here. ).

So, in order to have both tree-sitter and ocamllsp working at the same time, I had to tweak my configuration a little bit.

lspconfig.ocamllsp.setup({
  filetypes = vim.list_extend(
    require('lspconfig.server_configurations.ocamllsp')
      .default_config
      .filetypes,
    { 'ocamlinterface' }
  ),
})

vim.cmd([[au! BufNewFile,BufRead *.mli setfiletype ocamlinterface]])

And now, I am blessed with a consistent syntax highlighting for my mli files.